This invention relates to a method of burring for forming a generally short cylindrical projected burr on workpieces such as a metal plate.
In a conventional burring method, a circular prepared hole is formed by way of punching. The peripheral portion of the prepared hole is formed into a cylindrical configuration by way of drawing so that a short cylindrical burr is formed. A tapping screw is usually threaded into the burr formed as described above in order that another part or member is secured to the metal plate.
In the above-described conventional method, however, the metal plate needs to be extended by way of drawing so that the burr is formed. Accordingly, the peripheral portion of the burr has a thickness smaller than the base metal plate, which often causes the thickness of the burr peripheral portion to be smaller than the depth of the screw thread. In such a case the screw needs to be replaced with one having a smaller depth of the screw thread. Consequently, a sufficient screw tightening force cannot be obtained, resulting in insufficient securing of the other part or member to the metal plate.